The future impact of traffic emissions on atmospheric ozone and OH has been
investigated separately for the three sectors AIRcraft, maritime SHIPping
and ROAD traffic. To reduce uncertainties we present results from an
ensemble of six different atmospheric chemistry models, each simulating the
atmospheric chemical composition in a possible high emission scenario (A1B),
and with emissions from each transport sector reduced by 5% to estimate
sensitivities. Our results are compared with optimistic future emission
scenarios (B1 and B1 ACARE), presented in a companion paper, and with the
recent past (year 2000). Present-day activity indicates that anthropogenic
emissions so far evolve closer to A1B than the B1 scenario.
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As a response to expected changes in emissions, AIR and SHIP will have
increased impacts on atmospheric O<sub>3</sub> and OH in the future while the
impact of ROAD traffic will decrease substantially as a result of
technological improvements. In 2050, maximum aircraft-induced O<sub>3</sub> occurs
near 80° N in the UTLS region and could reach 9 ppbv in the zonal
mean during summer. Emissions from ship traffic have their largest O<sub>3</sub>
impact in the maritime boundary layer with a maximum of 6 ppbv over the
North Atlantic Ocean during summer in 2050. The O<sub>3</sub> impact of road
traffic emissions in the lower troposphere peaks at 3 ppbv over the Arabian
Peninsula, much lower than the impact in 2000.
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Radiative forcing (RF) calculations show that the net effect of AIR, SHIP
and ROAD combined will change from a marginal cooling of −0.44 ± 13 mW m<sup>−2</sup>
in 2000 to a relatively strong cooling of −32 ± 9.3 (B1) or
−32 ± 18 mW m<sup>−2</sup> (A1B) in 2050, when taking into account RF due to
changes in O<sub>3</sub>, CH<sub>4</sub> and CH<sub>4</sub>-induced O<sub>3</sub>. This is caused
both by the enhanced negative net RF from SHIP, which will change from
−19 ± 5.3 mW m<sup>−2</sup> in 2000 to −31 ± 4.8 (B1) or −40 ± 9 mW m<sup>−2</sup>
(A1B) in 2050, and from reduced O<sub>3</sub> warming from ROAD, which is
likely to turn from a positive net RF of 12 ± 8.5 mW m<sup>−2</sup> in 2000
to a slightly negative net RF of −3.1 ± 2.2 (B1) or −3.1 ± 3.4
(A1B) mW m<sup>−2</sup> in the middle of this century. The negative net RF from
ROAD is temporary and induced by the strong decline in ROAD emissions prior
to 2050, which only affects the methane cooling term due to the longer
lifetime of CH<sub>4</sub> compared to O<sub>3</sub>. The O<sub>3</sub> RF from AIR in 2050 is
strongly dependent on scenario and ranges from 19 ± 6.8 (B1 ACARE) to
61 ± 14 mW m<sup>−2</sup> (A1B). There is also a considerable span in the net
RF from AIR in 2050, ranging from −0.54 ± 4.6 (B1 ACARE) to 12 ± 11
(A1B) mW m<sup>−2</sup> compared to 6.6 ± 2.2 mW m<sup>−2</sup> in 2000.